If Meg's suggestion doesn't fix it:
Try resetting the preferences:
FAQ: How to reset preferences in Premiere Pro?
If that doesn't work, try resetting the Workspaces:
Reset a workspace
Reset the current workspace to return to its original, saved layout of panels.
- Do one of the following:
- Click the Workspace menu icon and select Reset to Saved Layout.
- Choose Window > Workspace > Reset to Saved Layout.
from here:
https://helpx.adobe.com/premiere-pro/using/workspaces.html
If that doesn’t work ,try updating or rolling back your graphics driver directly from the video card manufacturer’s site.